Coplanar strip transmission lines have been generally utilized as transmission lines, which feed a signal to a planar antenna or transmit a signal received by a planar antenna when the planar antenna is utilized for communication using a microwave or millimeter-wave band.
A transmission line transition, which has been utilized to make conversion from a microstrip transmission line to a slot transmission line possible and conversion from the slot transmission line to a coplanar strip transmission line possible, is shown in FIG. 6. In the example shown in FIG. 6, a first dielectric substrate 21 has an electromagnetically coupling conductor 24 for a coplanar strip transmission line disposed in a substantially dew-shaped form thereon, and the first dielectric substrate 21, a dielectric layer 27, a grounding conductor 26 and a second dielectric substrate 22 are laminated in this order. The second dielectric substrate 22 has the grounding conductor 26 disposed on a surface thereof close to the dielectric layer 27, and the grounding conductor 26 has a linear slot 25 formed therein. The second dielectric substrate 22 has an electromagnetically coupling conductor 20 for a microstrip line disposed on a surface thereof remote from the dielectric layer 27. All parts in the example shown in FIG. 6 except for the second dielectric substrate 22 and the electromagnetically coupling conductor 20 for a microstrip line are disclosed in “Microstrip Lines and Slotlines”, Second Edition, p. 440-441, 7.7.5 CPS-to-Slotline Transitions, coauthored by K. C. Gupta, Ramesh Garg, Inder Bahl, Prakash Bhartia. However, there has been a problem that a transmission line transition, which partly utilizes the prior art, is not suitable for miniaturization.
Additionally, the above-mentioned prior art reference is silent on specific dimensions of the electromagnetically coupling conductor 24 for a coplanar strip transmission line and the like.